Rett syndrome (RTT), caused by mutation of the DNA binding protein MECP2, is one of the most common causes for mental retardation in females. Both loss of function mutations of the gene as well as overexpression such as seen in the MECP2 duplication gain of function syndrome, lead to RTT-like syndrome indicating that increased MECP2 levels can be equally detrimental to the nervous system as MECP2 deficiency. While it has been well established that MECP2 deficiency causes RTT in a cell autonomous manner, recent evidence points to an additional cell-non-autonomous mechanism based on therapeutic effects of MECP2 expression in astrocytes or on transplantation with wild type microglia. These observations as well as the recognition that re- expression of MECP2 or treatment with small molecules can halt disease progression and can even revert symptoms in the adult Rett mouse suggest that MECP2 is required for the maintenance of neuronal function. While these results are exciting and suggest a rational treatment in humans, it is crucial to assess therapeutic strategies in well-defined experimental systems using human cells as readout. This project seeks to set up a platform that utilizes human RTT neurons for clarifying the roles of MECP2 in gene expression and that permits the evaluation of candidate treatments in culture as well as under in vivo conditions. Using human iPS cell- derived neuronal cultures, the initial goal is to establish an experimental paradigm that allows defining the molecular role o MECP2 in gene regulation and to provide a robust and quantifiable disease-relevant phenotypic readout in human mutant neurons. We will use molecular approaches such as CHIP-seq to map binding sites of MECP2 to 5mC and 5hmC modified genomic sites and dissect the modes of gene activation and repression. Furthermore, we will identify target genes of MECP2 and MECP2- interacting partners and clarify the deregulation of MECP2 target genes in loss and gain of function RTT. A major focus of the proposal is to establish a platform that allows assessing the efficacy of therapeutic strategies to reverse the RTT phenotype of human neurons under in vitro and in vivo conditions. (i) To overcome limitations of conventional neural 2D culture systems we will use RTT ES or iPS cells as starting point to generate human cerebral organoid cultures. This will enable the analysis of cell-cell interactions and of potentia therapeutic agents in a well-defined 3D test system. (ii) We will transplant GFP-marked neuronal precursors into the developing mouse brain to generate animals that carry human MECP2 mutant neurons incorporated into their brain. By allowing the human neurons to integrate into the intact mouse brain, we seek to establish a clinically relevant platform to perform in vivo validation of growth factors and small molecule compounds that could be beneficial for the treatment of RTT patients.